On November 15, 2017 with a fairly new iPhone X I encountered the same problem. After spending a lot of time on the phone with a higher level of tech-support the only thing that resolved the problem was completely wiping the phone restoring it to new conditions. I was then able to return a back up that had been made with my previous phone.
The problem repeated itself in late December and I’ve since spent more time on the phone with tech support who say that the current solution is now to wipe the phone to factory condition settings and load back my apps one by one Until I find the app that could be creating the problem.
That of course is totally unacceptable given the level of work that would be involved in reconfiguring everything including the Alexa app Google Home, devices that work with Apple home, bank accounts etc. it’s just something I’m not willing to do. I will forgo using Siri and concentrate on the future more with Google Home and Amazon Alexa